Electrical potential indicator



A ril 20, 1965 Filed Jan. 25, 1960 R. E. DE MOTT 3,179,847

' ELECTRICAL POTENTIAL INDICATOR 2 Sheets-Sheet .l

f 5 .INVENTOR.

April 20, 1965 R. E. DE MOTT 3,179,847

ELECTRICAL POTENTIAL INDICATOR Filed Jan. 25. 1960 2 Sheets-Sheet 2INVENTOR. FQQIeT/F DE/Yarr United. States Patent 3,1'7,847 ELECTRICALPOTENTIAL INDECATGR Robert E. De Mott, Delia, Alberta, Canada Filed Jan.25, 1966, Ser. No. 4,327 1 Claim. (Cl. 315-185) My invention relates tonew and useful improvements in electrical potential indicators, morespecifically to indicators adapted to be used with incandescent bulbsand the like either individually or connected in a series, the principalobject and essence of my invention being to provide a device of thecharacter herewithin described wherein a predetermined amount ofpotential may be indicated when such potential has been applied acrossthe lamp filament.

In the past, particularly when it is desired to ascertain which of aplurality of series-connected incandescent lamps have failed, it isnecessary to remove each lamp independently and replace same with a lampwhich is known to operate until the burned out lamp is located.

I have overcome this disadvantage by providing a potential indicatorincorporated in parallel with the filament of the said lamp whereby avisual inspection of the lamp enables the user to ascertain which of thelamps has failed. I accomplish this either by a color change substanceassociated with the said lamp together with a heat generating impedanceor by a deposit forming substance also activated by a heat generatingimpedance, or by the provision of two evacuated, sealed and joinedenvelopes, each one having a filament mounted therein, said filamentsbeing connected in parallel. One of these filaments is the mainilluminating filament and the other is an auxiliary filament adapted tohave a longer operating life and to operate at a lower temperaturethereby acting as an indicator.

It can therefore be said that the present invention relates toimprovements in electrical potential indicators and more particularly tosuch indicators for denoting a predetermined amount of potential, andfurther to improvements in such indicators for denoting when apredetermined amount of potential has been applied across anincandescent lamp or across any specific electrical device. With such alamp theextinguishing of same may be caused by the failure of itsfilament or by the interruption of the line potential.

One object of this invention, when in connection with incandescent lampsor electrical devices, is for denoting when either such a lamp or deviceis defective.

Another object of my invention is to utilize same in connection with anassociated means for indicating when such a lamp or device in serieswith a plurality of lamps or devices has failed, and to indicate whichparticular lamp or device has failed.

A further object of my invention is to provide a device of the characterherewithin described which is adapted to be used in connection withelectrical fuses whereby the fuse concerned may be isolated readily ifsame has failed.

Certain types of electric lamps and devices are frequently operated inseries, whereby the combined poten- In my invention an indicator willshow which of the lamps or devices in series has failed, without havingto test each lamp or device in the circuit. However, it is to beunderstood that the present invention is not limited to use With seriesconnections only, nor to use in lamp and tube circuits only. Also it isto be understood that two filaments in parallel have conventionally beenemployed sealed within the same lamp envelope, one filament supplyingthe normal illumination, and the other auxiliary filament operating at alower temperature, and having a longer operating life, and acting as anindicator filament. In this conventional arrangement should the lampenvelope develop a crack, voiding its vacuum, the said indicatorfilament and said main filament and said main filament would burn out,whereas with my improved invention this disadvantage is overcome.

As an advantage I provide an inexpensive indicator in association withvarious electrical apparatus, said indicator being mounted on theexterior, in the interior, or other locations in connection with suchlamps of devices, said indicator being electrically connected inparallel with a currenhcarrying element of said device Where indicationis desired, upon the application of a predetermined potential in theparticular circuit, said potential being greater than normal operationalpotential.

As a further advantage I provide an inexpensive indicator in associationwith a source of electrical potential where indication is desired, upontr e application of a predetermined increase in the amount of potentialand Where said increased potential results in an increase of saidindicators temperature.

As a means of mechanical and safety protection shielda heat responsivecolor change substance to be employed as an indicator, associated with aheat generating electrical resistor, and electric power source and anincandescent lamp filament or a current-carrying element or" anelectrical device. Said heater resistor being associated with said colorchange substance to be electrically connected in parallel with the lampfilament or said currentcarrying element, or across said source or"electrical potential.

in another embodiment my invention comprises a fusible and visibledepositable substance to :be employed as an indicator instead of a colorchange substance associated with the heat generating resistor. In thecase of each of the above embodiments the color change and the fusingand visible deposit occurs only When said substance is subjected to apredetermined increase in temperature as the result of said heaterresistor being subjected to a predetermined increase in the amount ofapplied electrical potential.

Sustained surge potential on a supply line is inlicated either by achange in color of said associated color change substance, or by a fusedcondition and a visible deposit of said associated fusible and visibledepositable substance.

In a series circuit, failure of a filamentof one lamp or acurrent-carrying element of one electrical device is indicated by aninoperative arrangement together with either a change in color of saidassociated color change substance, or a fused condition and a visibledeposit of said associated fusible and visible deposit-leavingsubstance.

In a series circuit, failure of line potential isindicated by aninoperative arrangement together with either unchanged color of all saidassociated color change substances or of unfused condition, in whichcase there re mains no visible deposit of any of said associated fusibleand visible depositable substances. t

In a further embodiment thereof, my invention comprises, in combination,two evacuated sealed and joined .9 envelopes, each one having a filamentmounted therein, the two filaments being connected in parallel one ofwhich is the main illumination filament, and the other an auxiliaryfilament to be employed as an indicator. Said auxiliary filament emitslight of a lower intensity than said main filament, and in comparisonhas a longer operating life, so that failure of the main filament isindicated by this lower intensity light emitted by said associatedauxiliary filament.

A series circuit failure of the main filament of such a lamp is hereindicated by the extinguishing of this main filament and an above normalintensity of light emitted by said associated auxiliary filament. At thesame time less than normal intensity of light is emitted by the intactfilaments of the other lamps of the series arrangement, and of coursefailure of line potential in such a series circuitextinguishes all thelamps.

With the foregoing objects in view, and such other objects andadvantages as will become apparent to those skilled in the art to whichthis invention relates as this specification proceeds, my inventionconsists essentially in the arrangement and construction of parts all ashereinafter more particularly described, reference being had to theaccompanying drawings in which:

FIGURE 1 is a side elevation of a lamp illustrating one embodiment ofthe invention mounted in the interior of the lamp envelope.

FIGURE 2 is a fragmentary elevation of the top half of a lamp envelopeshowing a further embodiment of my device mounted externally of theenvelope.

FIGURE 3 is a fragmentary elevation of the lower portion of a lampillustrating a further embodiment of my invention mounted within anelongated, transparent lamp base assembly.

FIGURE 4 is a perspective view illustrating the associated color changeor fusible and visible deposit substance applied on the heater resistorper se.

FIGURE 5 is a top plan section taken along the lines 5-5 of FIGURE. 1.

FIGURE 6 is a diagrammatic representation illustrating a plurality oflamps in series.

FIGURE 7 is a top plan view of FIGURE 2 along the lines 77 thereof.

FIGURE 7A is a side elevation of the cover per se of FIGURES 2 and 7.

FIGURE 8 is a side elevation of a lamp illustrating a further embodimentof my device.

FIGURE 9 is a side elevation of a lamp in phantom showing a yet furtherembodiment of my device.

' FIGURE 10 is a side elevation of the lower half of a lamp shown inphantom as illustrated in FIGURE 9 showing only the position of thetransparent area through V which the indicator may be viewed.

FIGURE 11 shows a side elevation of the upper portion of a lamp with afurther embodiment of my invention mounted externally thereof.

FIGURE 12 is a diagrammatic representation of a plurality of lampsmounted in series with the associated lamp indicators enclosed withinseparate evacuated envelopes placed upon the main evacuated envelopes.

In the drawings like characters of reference indicate correspondingparts in the different figures:

Proceeding therefore to describe my invention in detail, referenceshould be made to the accompanying drawings inwhich I have indicated alamp 1 having an evacuated and sealed envelope 2 and an incandescentfilament 3 supported by conventionally sealed-in lead wires 4. Theenvelope Z has a conventional base 5 attached thereto and is providedwith contact elements 6 and 7 therein to which contact elements, thewires 4- are connected conventionally. The wires 4- are also connectedto opposite ends of said filament 3 and are maintained in the desiredspaced relationship by means of vitreous head 8.

Dealing now specifically with the embodiment illustrated in FIGURE 1, Ihave provided a further pair of all lead wires 4' extending upwardlyfrom the lead wires 4, being maintained in spaced relationship byvitreous bead 9. Adjacent the tip 2' of the envelope, these extendedlead wires 4' are connected to the terminals of a heat generatingelectrical bleeder resistor 10 which is associated with a heatresponsive color change substance 11 supported upon a wafer 13. Thissubstance 11 acts as the indicator and is capable of undergoing a changein color only when subjected to a predetermined amount of heat, saidpredetermined amount of heat being produced by the heat generated bysaid resistor 10 and only when this resistor is subjected to apredetermined increase of potential thereacross.

The design and proportion of said heater resistor 1t) and said colorchange substance 11 should be such that the former assumes heatgenerating qualities, and the latter undergoes its color change only asa result of a predetermined temperature being generated as mentioned, asa consequence of a predetermined electrical potential being appliedacross said resistor 10. The amount of said predetermined temperaturefor this color change should be somewhat greater than the temperaturegenerated by said resistor 10 when it is subjected to only the normalpotential of said filament 3. As a result the amount of saidpredetermined electrical potential required for this color change willalso be somewhat greater than the normal potential of said filament 3.

Said color change substance 11 thus will be under the control of theheat generated by resistor It and should be such that a resultant colorchange will occur only when resistor 10 becomes suliiciently energizedto assume heat generating qualities and thus the generation ofsu'ificient heat will occur only when resistor 10 is subjected to apredetermined greater than normal amount of electrical potential totherefore indicate the condition of said associated lamp 1 as forinstance when filament 3 fails.

To provide a window through which said color change substance 11 can beobserved directly opposite same, the tip of said envelope 2 istransparent as indicated at 12. It said lamp l is to receive adecorative coating, said transparent area 12 should be left uncoated.

To determine the condition of said lamp 1 it is only necessary to lookat this area 12 to observe the condition of said associated color changesubstance 11. The heater resistor lit) should be of low wattage, andrelatively high resistance and consists of any suitable type, such asprinted, impregnated, carbon, metallized, sprayed, painted, wirewound orin filament form, and the like. The color change substance 11 to becomposed of any suitable substance, such as paint, invisible ink, dye,lacquer, a mixture of an oxidizing agent with a reducing agent and thelike, any of which would be capable of undergoing a change in color onlywhen subjected to a predetermined increase in temperature. This colorchange substance is to be'applied in any suitable manner, such as bycoating, by impregnating or by mixing with said resistor material, etc.Said color change coating may be applied by any suitable process, suchas by dipping, spraying, printing, painting and the like.

It is to be understood that the heater resistor It) and said colorchange substance 11 could conveniently be mounted on a wafer 13 (asillustrated in FIGURE 5) by printing, spraying, painting and the like ina similar manner to that employed in conventional printed circuits.

A Wafer such as this would consist of any suitable heat resistingmaterial, such as ceramic, mica or the like, and would be supported onthe interior surface of said envelope 2 near the tip at 14, similarly aselectronic vacuum tube electrode assembly mica support-wafers areconventionally supported. This wafer 13 also serves as a shield,preventing the normal heat generated by said filament 3 from producing a'color change in said substance 11. If preferred, the edge of wafer 13may be attached and bonded to the interior surface of envelope 2 at 14similarly as an incandescent lamp base is attached and bonded to itslamp envelope.

FIGURE 6 shows diagrammatically a number of lamps in series, eachsimilar to those illustrated in FIGURE 1 and embodying the principles ofthe present invention, with lead wires connecting with a source ofpotential.

When said filaments 3 are intact the high resistance paths of saidresistors it are in parallel with the relatively low resistance paths ofsaid filaments 3. As a result of this a small fraction of the linepotential which is insuflicient to cause color change is applied acrosseach of said resistors 10. However, when filament 16 of one of the lampsin FIGURE 6 is ruptured a change in color 17 of substance 11 associatedwith resistor it of that lamp is effected, as a consequence of saidresistor it? and said lead wires 4' being subjected to the resultantincreased potential incident to said filament failure, to indicate thecondition of said associated lamp 1. The amount of potential impressedacross the intact filaments 3 of the remaining lamps in the series isinappreciable, due to the small amount of current flowing through saidresistor lit which produces said color change 17. Consequently filaments3 of said remaining lamps are inoperative, as there is no appreciableheating of their filaments.

It is therefore, obvious that to determine the defective lamp 1, whensaid series of lamps cease to function, it is only necessary, to observethrough the transparent area at 12 of each lamp it to discover the onewith said changed color substance 17.

Although said heater resistors it? are at all times connected inparallel with the filaments 3 in said series arrangement theirrelatively high ohmic value and low wattage, together with their heatgenerating qualities only i (when under abnormal conditions) wouldresult in a minimum amount of current flowing therethrough. The wattageof each said resistor ill may be approximately in the realm of A Watt asa maximum. Hence the energy consumed by the bleeder circuit isinappreciable, and may be further reduced by employing types ofresistors It which have a negative resistance temperature coefiicient.The form and color change embodiment of the invention as illustrated inFIGURES 2, 4, 5 and 7 is the same as that previously described withrelation to FIGURES l and 6 except that here the resistor iii and colorchange substance ll, are mounted on the exterior of lamp 1 and on thetip of said envelope 2. In this location the resistor Ill may beelectrically connected across extended lead wires 4 which are heldtogether by a vitreous bead 9 as stated, and which sealably extendthrough the wall of said envelope 2 to the exterior of same.

As a means of safety and mechanical protection a cover It; is attachableas indicated at 19 over said associated color change substance ill andheater resistor it) to said envelope 2. This cover 13 as seen in FIGURES7 anl 7A has the aforementioned transparent observation area 22 directlyregistering over color change substance ill and must withstand the heatgenerated by resistor lit) when at full line potential, and ispreferably attached (as at 19) as a lamp base and lamp envelope areconventionally attached. In order that cover 13 and envelope 2 may besecurely bonded together, the cover may be attached to the envelopeprior to any decorative coating being ap plied to the exterior or" suchan envelope.

The form and color change embodiment of the invention as illustrated inFIGURE 3 is the same as the other embodiments except that in thisinstance the heater resistor lit and color change substance iii arelocated in the transparent part 23 of an elongated base assembly 21veniently be attached to said base (as at 22) as a lamp base and lampenvelope are conveniently attached.

Said resistor 10 and color change substance 11 could be mounted on thetransparent internal surface of said thimble 23 similar to the wafermounting illustrated in FIGURE 5. Said color change substance 11 is tobe applied on the transparent internal surface of the thimble 23, andheater resistor it is to be mounted on that surface of substance U. thatis non-adjacent to the internal surface of thimble 23. The resistor It)is electrically connected across said lead wires 4 as at 24, similar tothe conventional printed circuit technique.

Alternatively, instead of consisting of a thimble 23 joined to base 5 asdescribed, said base assembly 21 could be composed of a transparentsleeve with said base 5 being crirnped about the exterior circumferenceof same similar to the conventional manner employed in household fusetechnique.

It is further suggested that the heater resistor iii with the colorchange substance 11 could be mechanically suspended from andelectrically connected across said lead wires 4, internally within thespace occupied by transparent part 23. at the bottom of said envelope 2and prior to the mounting of the assembey in envelope 2. Saidtransparent part 23 of the assembly 21 is to serve for attachment ofbase 5 to same as at 22, and also for attachment of base assembly 21 tosaid envelope 2. This could be accomplished, following the assembly ofthe mount and envelope 2;, by properly positioning the envelope and base5 for placement in a mould and filling the intervening space between thebottom of the envelope and the top of the base with a transparentplastic so that base 5 is joined to part 23 as at 22.

Instead of being suspended from lead wires 4, said heater resistor litand color change substance 11 could be internally mounted withintransparent part 23 on a stem extension of the amount at the bottom ofenvelope 2.

The form and color change embodiment of the invention as illustrated inFIGURE 8 is the same as those previously described except that here theheater resistor it? and color change substance 11 are mounted on the tipof a heat resisting insulating stem 25, in the interior of envelope 2and near the tip of said lamp 1.

The resistor 16 could as Well consist of any suitable conventional typeof printed, sprayed, painted or the like conductor, and could beelectrically connected across extended lead wires 4 with resistor litand color change substance lllbein mounted on the tip of said stem 25 ina similar manner to the wafer mounting illustrated in FIGURE 5. Theextended lead wires 4' could also be of the printed type and impressedon the surface of stem 25 and electrically connected to said lead wires4 as at 24, as in conventional printed circuit technique.

The form and color change embodiment of the invention as illustrated inFIGURES 9 and 10 is the same as the previously described embodimentsexcept that in this instance the heater resistor 1i? and said colorchange substance iii are mounted on the internal surface of the glass ofenvelope 2 itself, similar to the wafer mounting illustrated in FIGURE5.

In this case as seen in FIGURE 10 the envelope 2 has the aforementionedtransparent observation area 12 disposed on the sideof same and thecolor change substance 11 would be mounted directly on the internalsurface of said envelope 2 and registering over said area 12. Theresistor Ill would be mounted on that surface of said substance ii thatis non-adjacent to the internal sur ace of envelope 2. The resistorlit), as aforementioned, is electrically connected across said leadwires4 at 24, similar to the conventional printed circuit technique.

' The form of the invention illustrated in FIGURE. 4 comprising heaterhesistor It and said color change substance Il can be electricallyconnected by leads denoted by 5 across the tcrminalsJlS of a source ofelectrical potential, where indication is desired upon the applica- 7tion of a predetermined increase in the amount of the applied potential.In order to determine the condition of said source of potential it isonly necessary to observe the condition of said associated color changesubstance 11.

In the embodiments of the invention employing a fusible and visibledepositable substance, it is to be understood that the same drawings andspecifications employed for the color change embodiments also apply tothe fusible and visible depositable embodiments. However, in this lattercase a substance is employed that is capable of fusing at a lowtemperature and leaving a visible deposit similar to the deposit left bythe conventional household fuse when same burns out. This fusible andvisible deposit-leaving substance would consist of any suitablematerial, such as wax, paint, celluloid, plastic, plastic cement,lacquer, bismuth, solder, fuse metal or the like, that is capable offusing at a low andpractical degree of temperature and leaving a visibledeposit. This substance is to be applied to said heater resistor It? inany suitable manner, such as by coating, by impregnating or by mixingwith said resistor material or in like manner. Said coating may beapplied by any suitable process, such as by dipping, spraying, printing,painting, etc.

In the two evacuated and sealed envelopes form and embodiment asillustrated in FIGURES 11 and 12, said potential indicator comprises alamp I having an evacuated and sealed envelope 2, an incandescentfilament 3 supported by lead wires 4, and mounted and sealed therein thethe conventional manner.

A pair of similar lead wires 4' which are joined to said lead wires 4are held together by a vitreous head 9, and extend upwardly through lamp1 and sealably extend exteriorly through the wall of said envelope 2. Inthis location, said extended lead wires 4' are electrically connectedtolead wires 29 and 30 that pass sealably into a separate evacuated andsealed envelope 311. In the interior of .this envelope 31, lead wire 29is electrically connected to one terminal of a small low wattagecurrentlimiting resistor 32. The other terminal of this resistor 32 iselectrically connected to one terminal of a small low wattage auxiliaryindicator filament 33. The other terminal of this filament 33 iselectrically connected to said lead wire 30. Thus the series connectedcombination (resistor 32 and filament 33) is electrically connected inparallel with said filament 3. The resistor 32 and filament 33 aresupported by said lead wires 29 and 36). The filament 33 will emit asubstantial intensity of light when said series connected combination ofresistor 32 and filament 33 is subjected to any predetermined increasein the amount of potential. As for example, if filament fails.

To be positively efiective the predetermined potential increase and thedesign of said resistor 32 and filament 33 should be such that filament33 has a longer life than filament 3. The filament 33 should be such asto assume light emitting qualities only, when a predetermined amount ofpotential is applied across the combination comprising resistor 32 andfilament 33, and hence only when a predetermined degree of temperatureis generated by resistor 32. At the threshold of light emission by saidfilament 33 said predetermined degree of temperature, as generated byresistor 32, would be greater than the degree of temperature generatedby the resistor itself when said resistor and filament combination issubjected to only the normal operational potential of the filament 3.Hence,

at the threshold of light emission by said filament 33 saidpredetermined amount of potential applied across said combination of 32and 33 would be greater than the normal operational potential of thefilaments.

To still further this accomplishment, resistor 32 and filament 33 shouldpreferably comprise substances having negative resistance temperatureco-efficients. The filament 33 would, therefore, be of such low wattageand low potential as to be only a fraction of that of said filament 3,together with relatively heavy cross sectional LI) area. Thus filament33 would have a longer operating life than filament 3.

It follows that light is emitted by said'filament 33 when resistor 32and filament 33 combination become sufiiciently energized for the saidfilament to assumelight emitting qualities. This occurs as a consequenceof said combination being subjected to greater than normal operationalpotential of a predetermined amount thereby indicating the condition ofsaid associated lamp 1. Under normal operating conditions, only thenormal amount of operational potential of said filament 3 is applied,across said combination of said elements, which would beinsufiicient tocause a substantial intensity of light emission by filament 33.

To determine the conditionof said lamp 1, it is only necessary toobserve the amount of intensity of light emitted by said auxiliaryfilament 33. When in a series circuit, failure of the main illuminationfilament 3 is indicated by an increase in the amount of intensity of thelight above the normal amount of operational intensity as emitted bysaid filament 33. Failure of line potential is indicated by the totalextinguishment of both of said filaments 3 and 33.

Said current-limiting resistor 32 may be of low wattage and relativelyhigh ohmic value, providing it has a negative resistance temperatureco-efiicient. It may consist of any suitable type conductor such asprinted, carbon, metallized, impregnated, sprayed, painted, wire-woundor the like. Instead of mounting resistor 32 in envelope 31, same couldconveniently be mounted along with said'main illumination filament 3 inenvelope 2. I

If desired, a transparent area 12 may be disposed on the tip of saidenvelope 31, so that the light emitted by said filament 33 can bedetected more readily.

This type of indicator may also be in the form of a unitary part of saidlamp 1 by having the envelope 31 attached to the envelope 2 as at 34 inthe same way that an incandescent lamp base is attached to its lampenvelope.

Any decorative exterior coating of lamp 1 could be applied followingthis attachment so as to ensure proper bonding between envelopes 2 and31.

In FIGURE 12 the lamps are identical to that illustrated in FIGURE 11,and similarly embody the principles of the present invention, and areconnected in series to wires 15 leading to a source of potential. One ofthese lamps in FIGURE 12 shows a ruptured filament 16, which results inan increased amount of intensity of light emission being apparent inenvelope 31 of that lamp. This is caused by the combined resistor 32 andfilament 33, with lead wires 29 and 36, being subjected to the resultantincreased amount of potential incident to the failure of this filament16, which indicates the condition of said associated lamp 1. The amountof potental impressed across the intact filaments of the remaining lampsis small, as a result of the small amount of current flowing throughsaid combination, all of which has produced increased intensity of lightemission. Therefore, said remaining lamps are inoperative as there is noappreciable heating of their filaments.

To determine the defective lamp 1, when said series circuit becomesinoperative and said lamps 1 cease to function, it is only necessary, toobserve the amount of intensity of light emitted by said associatedauxiliary filament 33 of each lamp 1, until the one with said increasedintensity of light emission 35 is found.

As mentioned, the current-limiting resistors 32 and auxiliary filaments33 are at all times connected in parallel with said lamps 1 in saidseries arrangement. As a result of the relatively high ohmic value ofthese elements when cold, together with low wattage and with saidfilaments 33 assuming light emitting qualities only (when under abnormalconditions) there would normally be'only a minimum amount of currentflowing theretthr-ough. Hence, the energy consumed by the bleedercircuit is small, and may be further reduced by employing types ofresistors 32 and filaments 3-3 which have negative resistancetemperature co-efiicients.

Since various modifications can be made in my invention as hereinabovedescribed, and many apparently widely different embodiments of same madeWithin the spirit and scope of the claim without departing from suchspirit and scope, it is intended that all matter contained in theaccompanying specification shall be interpreted as illustrative only andnot in a limiting sense.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

'In an incandescent lamp, the combination of an envelope, a filament, aheat generating impedance element connected in parallel with saidfilament to a source of current, said filament and said impedanceelement located within said envelope, said impedance element generatinga relatively small amount of heat when said filament is in operation butgenerating a relatively large amount of heat when said filament isinoperative and current passes through the impedance element only, and aheat-responsive color-changeable indicating member disposed adjacentsaid impedance element, said indicating member having a predeterminedcolor which is unaffected'by the relatively small amount of heatgenerated by said impedance element when said filament is in operation,but which color is permanently changed by the relatively large amount ofheat generated by the impedance element when the filament isinoperative.

References Cited by the Examiner UNITED STATES PATENTS 1,838,716 12/1311Stone 313-40 1,972,125 9/34 Anderson 315 1s5 2,152,228 3/39 Waters"315-185 2,461,962 2/49 Carlson s15 -1s5 2,489,649 11/49 Kuever 315-1s52,870,377 11/59 Ovrevik 315--185X ARTHUR GAUSS, Primary Examiner. RAL-PHNILSON, DAVID J. GALVIN, Examiners.

